Polyether-polyamide composed of a polyether moiety and a polyamide moiety is excellent in oil resistance and low-temperature impact resistance, and its application as an engineering thermoplastic elastomer has been increasingly broadened.
One of disadvantages of the polyether-polyamide is that the polyether moiety is liable to deterioration by oxidation. As a result, the main chain is cut to reduce the molecular weight, which leads to reduced mechanical properties.
Further, it is known that heating of polyether-polyamide to high temperatures for melt molding causes disadvantages, such as foaming, coloring, and surface roughening of molded articles.
Attempts have been made to minimize these disadvantages of polyether-polyamide by addition of a stabilizer as disclosed in Japanese Patent Application (OPI) Nos. 16756/78 and 49061/85 (the term "OPI" as used herein means "unexamined published Japanese Patent application", but it is also known that these techniques have their own limits.
In order to essentially solve the problems, block polyester-polyamide has been proposed, in which the polyether moiety of polyether-polyamide is replaced with a polymer having similar properties, such as an aliphatic polyester.
However, mere replacement of the polyether moiety of polyether-polyamide with a polyester to form block polyester-polyamide results in deterioration of water resistance. Besides, when the block polyester-polyamide contacts with a water content, it cannot hold its initial block copolymer structure composed of a polyester moiety and a polyamide moiety due to ester-amide exchange reaction during melt molding and, in some cases, eventually changes to a random copolymer. Such being the case, the polyester-polyamide does not maintain properties as a thermoplastic elastomer any more.